If you have already used soft scrub then not much else will hurt fiberglass. I have used just about everything out there with varying to little success UNTIL I got the nerve to try snobol toilet bowl cleaner. For $1.39 at wally world it has worked better than anything sold at West Marine, E&B, Wally World, etc. Wipe it on, let it sit for a little while then hose it off. Then you must put on a couple of good coats of wax or you will be doing this again. I use the Teflon wax and love it on my P19 which looks as white as an egg!